Temporal investigations of transient fuel spray characteristics from a multi-hole injector using dimensionless analysis

• Formation of high pressure spark-ignition direct-injection fuel spray is investigated from a force analysis point of view.
• Spray characteristics are governed by the competition among inertia force, viscous force, surface tension force, and aerodynamic force.
• Two temporal stages are identified during the development of spray formation.
• Transient spray characteristics are well correlated with dimensionless numbers including Reynolds number, Weber number, and gas-to-liquid density ratio.

The breakup of high speed liquid jets and the spray formation from a multi-hole direct-injection injector were investigated by analyzing the temporal development of spray penetration and cone angle. Planar Mie scattering was applied to generate spray images under a wide range of operating conditions with ethanol, methanol and gasoline as test fuels. Dimensionless parameters such as Reynolds number (ReRe), Weber number (WeWe), and gas-to-liquid density ratio (ρa/ρlρa/ρl) were used to enumerate the competition and balance of forces that dominated the spray formation, including the inertia force, viscous force, surface tension force and aerodynamic force. The results show that two temporally sequential stages exist during the spray formation, namely the initial stage and developed stage. During the initial stage, the spray penetration increases linearly with time after the start of injection (ASOI). The penetrating speed during the initial stage is primarily controlled by the competition between the inertia and surface tension while the aerodynamic force shows only minor influence. The duration of the initial stage of penetration is dependent on the competing process between inertia and viscous force as well as that between inertia and aerodynamic force. During the developed stage, the effect of aerodynamic forces becomes more influential on the spray penetration. The viscous force shows weak impact on developed stage penetration under low Reynolds number (Re < 12,500) conditions. In contrast with the strongly time-dependent penetration, the plume angle of the emerging jets stays relatively constant during the entire injection duration. Based on the above analysis, new dimensionless correlations have been established to quantitatively characterize the effects of competing forces on spray penetration and cone angle. Compared to the classical correlations, these new correlations explicitly express the relative importance of each force during different temporal stages of spray formation. Therefore, they provide more insight into the physical mechanism of high pressure spray formation process.